Tire repair operations, such as tire retreading operations, are generally used to extend the useful service life of a tire. Typical tire retreading operations include removing previously worn tread from a tire and bonding new tread in its place. Tires may be retread or repaired one or more times as a less expensive alternative to purchasing new tires, providing particular advantages for large-scale operations such as trucking, bussing and commercial aviation.
Generally, some level of non-destructive testing (NDT) of the tire prior to repair is conducted to determine whether it is appropriate to perform the repair operation. Visual inspection methods can be used to validate the integrity and, subsequently, the viability of retread and/or repair of tire casings for retread. For instance, the inside and outside surface of a tire can be visually inspected by an operator using special lighting to inspect for defects such as crazing, cracks, snags, bulges, depressions, gouges, abrasions, marbling, bubbles, blisters, separations, and other defects. Visual inspection methods, however, are subjective, inconsistent, and can require extensive training. Moreover, due to high operator turnover, difficulty exists in retaining expertise.
High voltage discharge (HVD) testing can be performed in place of or supplemental to visual inspection. HVD testing can be used to identify anomalies in the inner liner of a tire that penetrate the insulating material of the inner liner. In HVD testing machines, the tread portion of a tire is typically disposed between a pair of electrodes across which a high voltage electrical potential is generated. The voltage applied across the electrode will cause electrical discharge at the location of a defect in a tire. U.S. Pat. No. 6,050,136, which is incorporated herein by reference for all purposes, for instance, discloses a HVD test machine that employs electrical discharging to detect defects in the inner liner of a tire.
On a traditional HVD test machine, the probe assembly typically includes a series of wire loops and small chains that are positioned to hang inside the tire in a manner to distribute high voltage from bead to bead on the inside surface of the tire. The correct width probe must be chosen for the tire size. The ground path for the discharge at an anomaly is provided by contact of the tread on a metallic driven roller. When the probe passes over an anomaly, an electrical discharge passes through the tread at the location of the anomaly to the metallic driven roller.
Traditional HVD test machines suffer from several disadvantages. For instance, the detection capability of typical HVD test machines depends on many variables, including the thickness and chemical makeup of the tread portion of the tire. In particular, in typical HVD testing machines, the electrical discharge must pass through the tread portion of the tire from the high voltage probe to the metallic driven roller which acts as the reference electrode for the HVD testing machine. Variations in tread thickness result in variations in resistance due to more or less dielectric material in the electrical discharge path. This variability may be further enhanced by variations in tread compounds, such as, for instance, variations in tread compounds used for winter tires and tread compounds used for summer tires. This variability can require higher voltage to be applied to the high voltage probe to generate an electrical discharge and can lead to reduced accuracy in the detection of anomalies by the HVD testing machine.
Thus, a solution is needed for automated HVD testing of tires that provides for more uniform detection capability of anomalies. A solution that is less susceptible to tire variables, such as the thickness and chemical makeup of the tread portion of the tire, would be particularly useful. A solution that provides for a reduced voltage to be applied to a high voltage probe would also be particularly useful.